| Magnetic liquid is a new type of magnetic sensitive smart material.Magnetic liquid exhibits many irreplaceable properties in magnetism,optics,acoustics and mechanics,which make it widely used in the fields of aviation,aerospace,chemical industry,machinery and medicine.Rheology of magnetic liquids is the common theoretical basis for these applications.In recent 5 decades,a lot of achievements have been made in the study of rheological properties of magnetic liquids.However,due to the extremely complex anisotropic structures formed in magnetic liquid under magnetic field,a unified microscopic theoretical model has not been established in this field.At the same time,macroscopic rheology experiments are not systematic,so it is urgent to integrate the research results obtained by different experimental methods.Even in the most well-studied magnetoviscous effect area,the widely used chain-like microscopic model is based on strong ideal assumption,which makes it impossible to analyze the results of macroscopic rheological experiments.As to the viscoelasticity,thixotropy and other complex rheological properties of magnetic liquid,the research is just beginning and the mechanisms on the formation and breakage of different types of microstructures is not clear.With a series of magnetic liquids of different carrier fluid viscosities and particle volume fraction prepared,this thesis deals with the rheological phenomenon caused by different microstructures of magnetic liquids.According to the research requirements of magnetoviscous and complex rheological properties of magnetic liquids,new experimental measurement methods and experimental schemes were proposed.The steady and dynamic mechanical responses of magnetic liquids under different temperatures and magnetic fields were systematically studied and the microscopic evolution mechanism was analyzed deeply.By integrating the theory and experimental results of magnetoviscosity,viscoelasticity and thixotropy of magnetic liquid,the understanding of complex rheological properties of magnetic liquid is expanded and deepened.The rheological properties of magnetic liquids were described by different models.Combined with thixotropy,a new method to define the yield stress of magnetic liquids is proposed.In addition,the rheological theory of magnetic liquid was combined with engineering application by the research on the effect of viscosity and magnetoviscous effect on the properties of magnetic liquid sealing liquid medium.In terms of magnetoviscous properties of magnetic liquids,with the macroscopic dynamics theory and microscopic model of magnetic liquid combined,the magnetoviscous effect of fluorocarbon based magnetic liquid under different temperatures,magnetic fields and shear rates was studied.Based on the analysis of viscosity-temperature curves of magnetic liquids under different shear rates,two microscopic influencing mechanisms of temperature on magnetoviscous effect are proposed.Specified experiments were designed to show the transition of the two mechanisms.Through the study on the magnetoviscous coefficient,yield stress and Masson number curves of magnetic liquids with carrier fluids of different viscosities under different magnetic fields,magnetic liquid with higher carrier viscosity is proved to own more significant non-Newtonian properties.It is suggested that the viscosity of the carrier fluid can influence the motion and configuration of internal structures in magnetic liquids by hydrodynamic effect.Regarding the viscoelasticity of magnetic liquids,the static and dynamic viscoelasticity of magnetic liquids were studied by creep and recovery tests and small amplitude oscillatory shear tests respectively.The influence of particle volume fraction,magnetic field,temperature and other factors on viscoelasticity of magnetic liquid was analyzed.The experimental results showed that magnetic liquids with particle volume fraction exceeding 3.74%exhibit linear viscoelastic properties under certain magnetic field strengths,while magnetic liquids with volume fraction 1.88%do not exhibit linear viscoelastic properties under any magnetic field.According to the experimental results,the significant viscoelasticity of magnetic liquids under small strain or stress is attributed to the reversible orientation change of the internal column-like structure and the separation process of the structures from the surface of measuring rotor.The complete frequency sweep curve of magnetic liquid was measured experimentally based on the principle of strain rate frequency superposition,which proved that magnetic liquids underwent different relaxation mechanisms at different time scales.A generalized Burgurs model was used to simulate the viscoelasticity of magnetic liquids and it is proposed that the relaxation process of microstructure in magnetic liquid is more complex than that of common viscoelastic fluid in creep and recovery tests.In regards to the yielding and thixotropy of magnetic liquids,the time dependent rheological properties of magnetic liquids were systematically studied through a series of experiments,including static and dynamic yield stress tests,viscosity bifurcation under constant stresses,hysteresis loop tests and three interval thixotropic tests.The yielding and thixotropy of magnetic liquids are combined to analyze their common microscopic mechanism,i.e.,the formation and breakage of different types of microstructures over time.The viscosity bifurcation of magnetic liquid can be observed under different stresses,magnetic field strengths or temperatures,leading to the redefinition on yield stress of magnetic liquid from a thixotropic perspective.The results of 3 interval thixotropy test showed that significant thixotropic recovery can be caused by the reconstruction of chain-like or column-like structures.A nonlinear structure model was proposed based on the characteristics of formation and breakage of structures in magnetic liquid.The model was used to simulate the time-dependent rheological characteristics of magnetic liquids and the results showed that the viscosity bifurcation of magnetic liquids can be successfully predicted.With respect to the influence of magnetic liquid viscosity on sealing liquid medium performance,the critical pressure and sealing life of magnetic liquids with carrier fluids of different viscosities were studied experimentally,based on the experimental platform and sealing structure which were specially designed and built.The failure mechanism of magnetic liquid seal is also analyzed.The experimental results showed that the viscosity of magnetic liquid is one of the decisive factors for its performance on sealing liquid medium.It is suggested that the viscosity of magnetic liquid has different influencing mechanisms on the instability of seal interface and the wastage of seal film.The influencing degree of the two mechanisms together determines the performance of magnetic liquid when sealing liquid medium.The research in this paper is of great significance to the improvement of experimental methods and theories of rheology of magnetic liquids,and is conducive to the quantitative,precise application and regulation of magnetic liquids. |
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